The present invention relates generally to cranes and more particularly to cranes having a powered counterbalance unit spaced a substantial distance from an independently supported boom structure.
Rearwardly spaced counterweight structures provide stability to a loaded boom by counterbalancing the tendency of a load to produce a tipping moment. In general, the rearward displacement of the counterweight is a critical factor in the determination of the limits of control and lifting capacity of a crane.
One such form of crane structure includes a self-pivoting boom mounted on a vehicle that is provided with an immovable extended trailer. This trailer supports a traveling counterweight that can be displaced outward on the trailer as the lifted load increases either due to load size or displacement of the load from the base of the boom. The operator is limited to a relatively small arc through which he may move a load, the length of arc on each side of the boom center being decreased as the size of the load is increased. Sometimes outriggers are used to broaden the base of a vehicular crane. Outriggers, however, tend to eliminate any possibility of mobility under load.
U.S. Pat. No. 3,836,010, granted to the present applicant, marked a substantial improvement in the heavy lift crane field. It disclosed a mobile crane structure with a horizontally rotatable load platform. The platform was rigidly connected to a rearwardly displaced counterweight unit. The distance between the two mobile base units was spanned by a rigid spreader link or stinger used to transmit motion of the mobile counterweight unit to rotational motion of the boom and staymast mounted on the load platform. Thus, a crane structure was provided that included a rotatable platform supporting a boom and staymast with great load lifting capacity and which was stabilized by a counterbalanced, earthborn mobile vehicular base that precisely and accurately moved the boom and staymast about a vertical axis defined at their supporting platform.
The present improvements were added to the patented structure to increase its over-all stability and efficiency of operation. For example, longitudinal forces applied along the length of the "spreader link" or stinger mounted between the pivoted load platform and the counterweight unit can be substantial unless the powered mobile unit is moved in a perfect circular arc about the center pivot axis of the platform. Furthermore, severe bending and torsional stresses can be applied if the counterweight unit shifts elevationally or moves over slightly rough terrain. It therefore becomes desirable to provide some form of interconnecting mechanism that will allow limited radial movement of the counterweight unit relative to the crane base and which accommodates slight elevational movement of the counterweight unit relative to the crane base. Allowances for such movement must be made without detracting from the ability of the counterweight unit to transmit turning forces directly to the main boom and staymast assembly for the purpose of pivoting the boom and load.
Another common problem is the tendency of an unloaded boom to recoil in a backward direction due to the moments applied to it by the overhead support rigging. It is common practice to use safety stops or cables between interconnected boom or mast members to limit the upward angle of the boom. The straps effectively prevent the boom from being pulled too high when unloaded but also limit the minimum radius at which loads can be lifted. Furthermore, such straps are not entirely effective when used in conjunction with single unit boom arrangements where the boom is supported directly at the crane base. In such cases, the boom is of substantial length and mounting of a safety strap or cable between the boom and base would be difficult, if not impossible, due to geometric restrictions between the boom and base.
When spacing between the boom base and counterweight unit is substantial to gain mechanical advantage at the end of the boom, a staymast is employed that extends rearwardly opposite the boom. The potential moment exerted on the raised, unloaded boom by the combined weight of the staymast and rigging often require safety stops or cables beyond practical strength limitations. One particularly serious problem can occur with structures using a backwardly angled staymast. If a heavy load is suddenly dropped or detached from the main boom, a backlash or recoil effect is produced causing a tendency for the boom to pivot upwardly toward the upright base pivot axis. This energy is often taken up before the boom reaches the upright orientation or passes over center due to the fact that the boom has a positive downward impetus opposite the direction of the recoil forces. The staymast, on the other hand, is subjected to a downward moment that is in the direction of the recoil. Thus, forces produced during recoil on the staymast are substantially greater. Limiting stops extending between the base of the staymast and spreader link or counterweight unit are impractical due to the potentially tremendous forces applied through the staymast. It is therefore desirable to provide some form of brace arrangement that will effectively support the outer end of the staymast in a stable condition regardless of loading, and that will not adversely affect performance characteristics of the crane.
The present crane structure includes mechanisms for accommodating limited relative movement between the crane base and counterweight unit, in addition to providing a counterweight strut extending between the counterweight unit and the outer end of the staymast. A telescoping, pivoted connection is provided between the mobile crane base and counterweight unit that permits both radial movement of the counterweight unit relative to the crane base and relative pivotal movement between them without undesirable loading at the connections along the length of their connecting stinger. A rigid counterweight strut extends between the staymast and the remote counterweight unit to replace the usual backstay lines or stops. It serves as a tension and compression member when the load is being applied or removed from the boom. It further functions to assist in the physical erection of the crane structure.